Each infectious disease is embedded within an ecological context. Disease ecology centers on investigating disease systems in order to understand the dynamics of disease transmission and what conditions promote or inhibit specific diseases.

Roughly 75% of all emerging infectious diseases are zoonotic—transmitted to humans by other creatures.

Diversity can reduce disease risk by reducing host abundance, diverting vector meals to hosts less competent at transmitting infection, reducing vector abundance, and reducing the rate at which pathogens are transmitted. Researchers have termed these mechanisms the dilution effect.

Conversely, in some cases, as species diversity declines, the chances that human beings will contract specific zoonotic diseases may increase.

Forest fragmentation diminishes species diversity. In Dutchess County, New York, this appears to be promoting the spread of Lyme disease. In Africa, the widespread disease leishmaniasis appears subject to some of the same mechanisms that spread Lyme disease.

Under some conditions, species diversity can increase disease risk. Careful investigations into specific disease systems are required to better understand this.